The needs of methods for making patterns of nanometric dimensions have increased sharply over recent years, due to the trend of miniaturisation of electronic components.
Initially, the patterns were made by optical projection lithography. In this production method, a photosensitive layer is deposited on a substrate, then exposed to an ultraviolet beam through a mask defining the patterns. The size of the smallest pattern that can be made (also called critical dimension “CD”) is closely linked to the wavelength of the beam used: the shorter the wavelength, the finer the pattern made and the higher the integration density of these patterns in an integrated circuit.
The ultraviolet beams used in photolithography traditionally have a wavelength of 193 nm or 248 nm. This method of defining patterns has the advantage of being well controlled and less expensive than other methods, especially electronic lithography methods. Nevertheless, with such wavelengths, the resolution of the exposure tool is limited.
In order to make finer, better resolved patterns, and thus further increase the integration density, new methods have been developed, such as double exposure (double-patterning) photolithography, extreme ultraviolet (EUV) lithography and electron beam (e-beam) lithography.
Among emerging lithography technologies, it is also possible to cite techniques of directed self-assembly (DSA) of block copolymers. Block copolymers are polymers in which two repeating units, a monomer A and a monomer B, form chains bound together by a covalent bond. When sufficient mobility is given to the chains, for example by heating these block copolymers, the chain A and the chain B have a tendency to separate into phases or blocks and to reorganise themselves under specific conformations, which depend especially on the ratio between the monomer A and the monomer B. As a function of this ratio, it is possible for example to have spheres of monomer A in a matrix of monomer B, or instead cylinders of A in a matrix of B, or instead intercalated lamellae of A and lamellae of B. The size of the domains of block A (respectively block B) is directly proportional to the length of the chains of monomer A (respectively monomer B). Block copolymers thus have the property of forming patterns which can be controlled using the ratio of monomers.
Known techniques of self-assembly of block copolymers can be grouped together into two categories, grapho-epitaxy and chemi-epitaxy, both described in detail in the article [“Guided self-assembly of block-copolymer for CMOS technology: a comparative study between grapho-epitaxy and surface chemical modification”, L. Oria et al., SPIE 2011, Vol. 7970-24].
Chemi-epitaxy consists in modifying the chemical properties of certain portions of the substrate, to force the organisation of the blocks of copolymer between these portions. These chemically modified portions of the substrate are typically delimited by a photolithography step followed by a plasma step.
Alternatively, grapho-epitaxy consists in forming primary patterns called guides on the surface of a substrate, these patterns delimiting areas inside which a layer of block copolymer is deposited. The guiding patterns make it possible to control the organisation of the blocks of copolymer to form secondary patterns of higher resolution inside these areas. The guiding patterns are conventionally formed by photolithography in a layer of resin, and potentially, transferred into a hard mask.
The technique of grapho-epitaxy has recently been used to form contact holes in an integrated circuit. After deposition and assembly of the block copolymer in the guiding patterns, the secondary patterns are developed by selectively removing one of the two blocks of the copolymer (for example the cylinders of A), thereby forming holes in the remaining layer of copolymer (the matrix of B). Then, these holes are transferred by etching onto the surface of the substrate, generally in a dielectric layer.
Thanks to this technique, it is possible to reduce the dimension of the patterns (application called “contact shrink”), the secondary pattern being of smaller dimensions than the primary pattern, i.e. the guiding pattern. There is then only a single contact hole per guiding pattern. It is also possible to multiply the number of patterns, by forming several contact holes per guiding pattern (“contact multiplication”).
As discussed in the article [“Etch challenges for DSA implementation in CMOS via patterning”, P. Pimenta Barros et al., SPIE Proceedings Vol. 9054, March 2014], current grapho-epitaxy methods are dependent on the density of the guiding patterns on the substrate. In fact, since the filling of the guiding patterns takes place by spin coating, the thickness of the block copolymer layer inside a guiding pattern depends on its surface area, its aspect ratio (depth/width) and the number of guiding patterns nearby. Consequently, for guiding patterns of same dimensions, the thickness of the copolymer layer in an isolated pattern is greater than the thickness obtained in a plurality of close together patterns.
Yet, the thickness of the layer of copolymer in the assembly guides affects the transfer of the patterns by etching, because the layer of copolymer serves as etching mask. If in certain assembly guides the thickness of copolymer is too low, an increase in the critical dimension of the patterns may occur during their transfer, because the etching mask is insufficient. Conversely, when the thickness of copolymer is too high, contact holes may be missing, their transfer having failed.
The thickness of the copolymer layer also affects the step of self-assembly of the block copolymer. In fact, a too low or too high thickness of the copolymer layer in the guides can lead to a poor organisation of the blocks. In particular, certain polymer patterns may not be oriented perpendicularly to the substrate. Generally speaking, these assembly defects concern isolated guides, where the thickness of block copolymer is the greatest.
Thus, since the thickness of the block copolymer layer varies within the guiding patterns of a same substrate (for example according to their density), it is rare to obtain the assembly and the transfer of all the secondary patterns with the same performances, especially in terms of critical dimension.